An oil-filled electrical device such as oil-filled transformer is structured in such a manner that a copper coil, which is a current-carrying medium, is wrapped with insulating paper for preventing the copper coil from being short-circuited between turns adjacent to each other.
It is known that a mineral oil used for the oil-filled transformer contains a sulfur component that reacts with a copper part in the oil to cause electrically-conductive copper sulfide to be precipitated on a surface of the insulating paper, and an electrically-conductive path is formed between turns adjacent to each other, resulting in a problem for example that dielectric breakdown occurs (for example, CIGRE T F A2.31, “Copper sulphide in transformer insulation”, ELECTRA, February 2006, No. 224, pp. 20-23 (Non-Patent Document 1)).
Recent studies have revealed that the presence of a specific sulfur compound that is dibenzyl disulfide (hereinafter referred to as DBDS) in an insulating oil causes copper sulfide to be precipitated on a surface of an insulator (for example, F. Scatiggio, V. Tumiatti, R. Marina, M. Tumiatti, M. Pompilli, and R. Bartnikas, “Corrosive Sulfur in Insulating Oils: Its Detection and Correlated Power Apparatus Failures”, IEEE Trans. Power Del., January 2008, Vol. 23, pp. 508-509 (Non-Patent Document 2)), and have also revealed a process in which copper sulfide is precipitated from DBDS (for example, S. Toyama, J. Tanimura, N. Yamada, E. Nagao, and T Amimoto, “High sensitive detection method of dibenzyl disulfide and the elucidation of the mechanism of copper sulfide generation in insulating oil”, the 2008 Doble Client Conference, Boston, Mass., 2008 (Non-Patent Document 3)).
It is being found that bibenzyl, benzyl sulfide, and toluene that are byproducts generated in the process of precipitation of copper sulfide originating from DBDS can be detected to determine the amount of precipitated copper sulfide from the concentration of the byproducts, since the amount of precipitated copper sulfide is proportional to the byproduct concentration in the insulating oil, and thus an abnormality of the oil-filled electrical device can be diagnosed. Non-Patent Document 3 describes the results of experiments on copper sulfide precipitated on a copper surface of a coil, because the experiments are conducted at a high temperature of 150° C. It is known that, at a temperature of 60° C. to 90° C. which is an operating temperature of the transformer, the copper sulfide precipitated due to dibenzyl disulfide (DBDS) is not precipitated on a copper coil but precipitated on insulating paper.
The inside of the oil-filled electrical device has a temperature distribution. In the case of the transformer, for example, the temperature distribution may include a temperature difference of approximately 20 K. Precipitation of copper sulfide has temperature dependence and precipitation occurs earlier at a higher temperature portion. Because of this, in a device having a temperature distribution, copper sulfide is not uniformly precipitated. The concentration of bibenzyl and toluene for example that is obtained from analysis of components of the insulating oil is proportional to the total amount of copper sulfide precipitated in the whole device. An abnormality of the device occurs at a high temperature portion where precipitation of copper sulfide concentrates, and therefore, diagnosis has to consider the temperature distribution in the device. Thus, there has been a problem that, even if a method in which the concentration of byproducts in the insulating oil is merely measured is applied to diagnosis of an actually operating device, the oil-filled electrical device cannot be diagnosed accurately, because the relationship between an abnormality of the device and detected byproducts is not clear.
It has long been known that copper sulfide is precipitated on a metal surface, which is a different phenomenon from the above-described precipitation of copper sulfide on a surface of insulating paper. In this case, when the amount of generated copper sulfide increases, the copper sulfide could peel off from the metal surface and then float in the insulating oil to deteriorate the insulation performance of the device. As a method for preventing this phenomenon, a method separately provides in the device a detection member with metal particles dispersed on the surface for the purpose of detecting generation of copper sulfide on the metal surface (for example, Japanese Patent Laying-Open No. 04-176108 (Patent Document 1)). This method can detect generation of copper sulfide from a decrease of the surface resistivity of the detection member to diagnose an abnormality of the device.
The diagnostic method disclosed in above-referenced Patent Document 1, however, relates to copper sulfide precipitated on a metal surface as has long been known, and this is a phenomenon different from precipitation of copper sulfide on a surface of insulating paper. Further, the method is accompanied by a problem that the detection member for detecting precipitation of copper sulfide has to be separately provided in the device.    Non-Patent Document 1: CIGRE T F A2.31, “Copper sulphide in transformer insulation”, ELECTRA, February 2006, No. 224, pp. 20-23    Non-Patent Document 2: F. Scatiggio, V. Tumiatti, R. Marina, M. Tumiatti, M. Pompilli, and R. Bartnikas, “Corrosive Sulfur in Insulating Oils: Its Detection and Correlated Power Apparatus Failures”, IEEE Trans. Power Del., January 2008, Vol. 23, pp. 508-509    Non-Patent Document 3: S. Toyama, J. Tanimura, N. Yamada, E. Nagao, and T. Amimoto, “High sensitive detection method of dibenzyl disulfide and the elucidation of the mechanism of copper sulfide generation in insulating oil”, the 2008 Doble Client Conference, Boston, Mass., 2008    Patent Document 1: Japanese Patent Laying-Open No. 04-176108